(335c) Elucidating Plasma-Catalyst Contributions for the Direct Conversion of Methane to Aromatics

The direct transformation of methane to liquid fuels and chemicals remains a critical research target due to the availability of natural gas and the abundance of methane in these resources. Methane dehydroaromatization is a promising route for the direct conversion of methane to aromatics (i.e., benzene, toluene, xylenes) under non-oxidative conditions. Among active metals studied for this reaction, Mo/H-ZSM-5 has been shown to be efficient toward C-H activation and formation of aromatic compounds. However, the commercialization of this process remains unfeasible, mainly due to the endothermic nature of this reaction (ΔH_R^o = + 532 kJ/mol), where equilibrium conversion is limited to 14% at 973 K. In addition, these conditions favor coke formation over the surface of the catalyst, leading to rapid catalyst deactivation. An alternative to the thermal conversion of methane to aromatics is the use of non-thermal plasma technologies. Non-thermal plasmas can facilitate chemical conversions of stable molecules (i.e., CH₄, CO₂ and N₂) at ambient pressure and temperatures through electron-impact reactions. Therefore, combination of the dehydroaromatization catalyst and non-thermal plasmas could potentially mitigate the limitations presented for this process.

In this work, we have addressed questions that remain unanswered regarding the role of the active metal (i.e., Mo) for the conversion of methane to aromatics in a one-pot dielectric barrier discharge reactor. We have specifically investigated the influence of the bulk gas temperature of the plasma-assisted reaction to elucidate the functionality of Mo and determine the contribution to plasma-phase reactions and Mo-catalyzed reactions. Our work shows that conversion of methane to aromatics can take place at temperatures as low as 573 K, where the multifunctionality of Mo/H-ZSM-5 is not required under these conditions. As the bulk gas temperature for the reaction is further increased, we observe additional contributions from Mo-catalyzed pathways toward the production of aromatics under plasma stimulation. This result is supported by the observation of Mo carbide species that are obtained at temperatures of 773 K in the presence of the plasma, which are not observed under thermal conditions using methane as a source. Overall, this presentation will highlight our current understanding of the role of plasma and catalyst for the conversion of methane to aromatics, and the influence of the bulk gas temperature over these complex plasma-catalyst interactions.